In many technical fields, capacitive sensors are used to detect the presence of certain substances. Generally, the capacitive sensor is arranged to measure a certain capacitance. If a substance is brought close to the sensor or in contact with the sensor, the dielectric conditions and the capacitance close to the sensor change, giving rise to a change in the capacitance value detected by the sensor. Based on the measured capacitance value, it thus is possible to e.g. detect the presence of fingertip brought close to the capacitors of the sensor. It is well known to use such capacitive sensors e.g. for the touch screen of computer terminal or a mobile device. Similarly, capacitive sensors are used e.g. as input devices for inductive heaters, e.g. modern cooking plates.
It is known from WO 2009/058359 A1 to use a plurality of capacitance sensors to determine the nature of a substance. The document U.S. Pat. No. 7,260,987 B2 describes the use of alternating voltage signals of differing frequencies for a capacitive fill level measurement.